Sports writer with The Age

Players have been warned they will need to be vigilant and stay within a ''safe level'' of caffeine intake as part of the AFL’s crackdown on supplements.

As part of the league’s new prohibited treatments list, players are not to have a caffeine level ''above the dose of 4mg/kg''.

Club doctors believe this limit could be reached with about five No-Doz caffeine tablets, although that could vary depending on a player’s size and tolerance to caffeine. It is this variance that doctors and players must be mindful of.

''You will need to take a fair bit to get above that prohibited level, but the trouble is, it’s a little bit risky in that people metabolise caffeine differently, so it’s bit hard to know where you are at unless you have tested for it yourself," one club doctor said.

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"They just need to be careful with it. The risk is the individual variation on the way things work.''

But players almost certainly won’t face sanctions for breaking the safe limit, as the AFL said: "There will be no urine testing for compliance but auditing of the doctor’s approval documentation.''

''The purpose of having a clear dose is so players are aware what are safe levels,'' AFL spokesman James Tonkin said. "This is worked out with the club doctor who will assess all sources of caffeine - foods, beverages, tablets, etc.''

Caffeine is not banned by the World Anti-Doping Agency, which believes it does not provide a performance-enhancing benefit.

On the WADA website, it says: "The status of caffeine has not changed from last year. Caffeine was removed from the prohibited list in 2004. Its use in sport is not prohibited.

"Caffeine is part of WADA’s monitoring program. This program includes substances which are not prohibited in sport, but which WADA monitors in order to detect patterns of misuse in sport.

"The 2010 and 2011 monitoring programs did not reveal global specific patterns of misuse of caffeine in sport, though a significant increase in consumption in the athletic population is observed."

But a significant issue has been the effect that caffeine has had on players in terms of trying to sleep after a match, particularly after a night game.

Concerns have been sparked because some players have turned to sleeping tablets to dim the effects of caffeine products. In 2010, Richmond's Ben Cousins was hospitalised and spent a night in intensive care after what the Tigers described as an "an adverse reaction to a sleeping pill".

Some players maintain an injection of caffeine is a crucial part of their pre-game build-up.